This invention relates to a method for the preparation of water-borne thermoplastic polyhydroxyethers having narrow polydispersity, coating compositions containing the same, and the use of such compositions for providing flexible coatings.
Coatings that contain no solvents have been prepared from very low molecular weight epoxy resins. The resin in the coating serves as the wetting agent and as the vehicle for any pigments or fillers desired. Because there is no solvent, coatings of this type tend to be free from pinholes, but have disadvantages such as brittleness, poor thermal stability and relatively high costs.
Coating compositions based on higher molecular weight epoxy resins have been prepared as solutions, formulated with solvent, curing agents, modifiers and pigments. These higher molecular weight epoxy resins are often in the form of an ester, obtained by reacting the epoxy resin with a fatty acid. While suitable for many purposes, epoxy ester-based coatings also have disadvantages such as being vulnerable to caustic attack.
In more recent years, there has been a trend toward water-based coating compositions containing epoxy resins, because of ease of handling and cleanup. One such resin is a modified epoxy resin made by grafting an addition monomer component onto the aliphatic backbone of the epoxy resin. For water-dispersible coatings, the addition polymerizable monomer is usually acrylic acid. Such water-dispersed epoxy coatings are particularly useful for the formulation of can coatings. These resins perform well when sprayed onto the surface of an already formed can body but have not found success for use in coating surfaces that are later formed into various shapes, including can ends. This is due to the inflexibility of the resins and their relatively low molecular weight. Increasing the molecular weight of the epoxy resin in order to achieve the desired properties results in the use of strong organic solvents, in order to dissolve the higher molecular weight resin, which must later be removed.
Another form of water-borne resins is based on thermoplastic polyhydroxyether resins, a family of amorphous, high molecular weight phenoxy resins derived from diphenols and epichlorohydrin. These "phenoxy" resins are known for their excellent flexibility, impact and abrasion resistance and adhesive strength. Because of these properties, they are used commercially for a variety of water-borne coatings. However, because of their high molecular weight and chain rigidity, again, a large amount of strong organic solvent must be employed to achieve a satisfactory applicable production process. To prevent pollution of the atmosphere, environmental standards require that such strong organic solvents be either recovered or burned at considerable costs.
The need exists for water-borne thermoplastic polyhydroxyether resin coating compositions that provide coatings having the desired toughness and flexibility for use on flexible substrates, while having low viscosities which do not require additional steps of solvent reclamation.